There is no possible way that iGo is loading every single road, building and POI of every single map into RAM all at once (plus DEM files, etc). It would just not be possible unless you had much more than 128mb of RAM! Being that all of the "content" files can easily take over 4GB of storage space, I would think that you would also need about the same amount of RAM if iGo actually loaded everything into RAM at once.
That being said, I have no idea how much iGo actually does load into RAM at the same time... Must be some sort of algorithm used to determine exactly what needs loaded into RAM - and it probably loads and unloads things as needed.
That's why I never really "bought" that the more content you have loaded onto a storage card means that you need more RAM. The more content you have may cause loading times to be longer, as iGo does at least need to identify what maps you have, etc, but it's definitely not reading and loading all content into RAM at once. No possible way it could do that with the limited amount of RAM installed in these GPS units.